The present invention relates to a transmission method and system using a standard transmission network for connecting elements of a seismic device, allowing to transmit with precision a signal indicative of a time break.
In the field of seismic prospecting signals received by seismic receivers distributed on the ground surface, which are reflected by discontinuities of the subsoil in response to the emission of seismic waves by a vibrational or impulsive source, are recorded. Seismic data collection sets comprising a large number of acquisition units distributed on the site to be explored and suited to collect (amplify, filter, digitize and store) the signals are used. The stored data are transmitted to a central control and recording station from each acquisition unit at fixed time intervals (for example after each emission-reception cycle, after each daily session, etc.) or, as soon as a transmission time interval is available. The transmission of stored data is either direct, or via intermediate stations or concentrators.
Seismic acquisition systems are for example described in U.S. Pat. Nos. 4,583,206, 4,628,494, 5,245,647, 5,550,787, 5,706,250, 5,563,847 and 5,822,273.
It is important that all the acquisition units distributed on the explored site can be synchronized with a common time break, generally the time of triggering of the seismic source, whatever the distance from the central station that transmits the indicative signal thereto and whatever the transmission channel used. Therefore, collection of centralized data can be very imprecise.
There are well-known methods and devices allowing perfect readjustment in time of distant stations to a central station communicating by means of communication channels, provided that it has been possible to measure the time of propagation of the signals on these channels.
U.S. Pat. No. 4,628,494, filed by the assignee describes, a method for synchronizing the acquisition of seismic signals by an acquisition unit in the field with a time break such as the time (TB) of triggering of a seismic source in the case where the time of propagation xcex94t thereto of the signal indicative of this time, via a transmission channel such as a cable or a radio channel, is known. The method essentially sends a pre-signal initiating the acquisition of the signals coming from the seismic receivers and their storage in a local memory. When the synchronization signal subsequently emitted is received by the acquisition unit at a time t, all the samples stored from the time (t-xcex94t), i.e. the time break, are sought in the local memory.
French Patent Application 98/15,792 describes a method and a device also allowing to produce, for each seismic signal, a series of samples of these signals, re-staggered from a time break, from a first series of digitized samples of the seismic signal produced from any time break prior to the time break, based upon knowledge of the effective time interval between these two times. The method comprises determining coefficients of a digital filter likely to compensate for the fractional part of the effective time interval measured, and applying this digital compensation filter to the first series of samples, thus allowing obtaining of a series of digitized samples re-staggered from the time break.
However, these known locating and readjustment techniques are applicable only if the time of propagation of synchronization signals through the emitting and receiving stations is known with precision. Transfers are, managed by software using multitask management with uncertainty about the effective propagation time in a seismic transmission system as mentioned above not exceeding about 50 xcexcs.
Transmission without a random time lag is possible and easier to implement when one owns the network that is used is owned by the party performing the test and in control of the emitted signals a coding mode is maintained. When the TB signal indicating the effective time of emission by the source reaches the central station, it is possible, via suitable circuits, to inject this signal directly into the communication channel that connects it to the receiving station, and thus to prevent possible non-constant delays due to the station management information system.
Standard communication networks are advantageous. They allow high-rate transmissions, they are relatively simple to use and open-ended. They however work according to a particular communication protocol with specialized pilot and control circuits which have to be adapted to in order to transmit signals representative of time breaks with precision.
This is the case in the system described in French Patent Application 99/12,113 filed by assignee which uses a standard communication network of Ethernet type to connect a central control and seismic recording station to dependent stations: local intermediate control and concentration stations, local acquisition units, and to transmit a synchronization signal (TB) thereto. The time of transit of the data on the transmission channels between the central station and the acquisition units is measured beforehand, acquisition of the seismic data by the acquisition units is pre-initiated; and the retention time interval in relation to the time of reception of the synchronization signal is fixed and greater than the transit time. An adjusted compensation delay is applied to the synchronization signal in the intermediate station in order to account for the fluctuations of the effective transfer time linked with its passage through a multitask control of the intermediate stations and the transmission network so as to respect the retention time interval. All the acquisition units can thus be perfectly synchronized.
This method is quite suitable when the time of transit via the transmission channel is constant and can be measured beforehand, which is the case with a standard communication network comprising material links (low-rate wired Ethernet network for example).
On the other hand, in any other case where the time of transit on the available channel of the standard network (radio link for example) is likely to fluctuate within a time interval incompatible with the precision required for transmission of a synchronization signal such as a TB, the compensation schemes of the prior system are inadequate.
The transmission method according to the invention uses a standard communication network for transferring synchronization data between a central control and seismic recording station and dependent stations (local seismic data acquisition, processing and transmission units depending on the central station either directly or via one or more intermediate local control and concentration stations, itself connected to local acquisition units), in the case when the transmission time on the connection channels between the elements is subject to great fluctuations.
The transmission system according to the invention has applications in many fields where distant stations have to be synchronized without necessarily using dedicated communication means.
In principle, the invention provides the communicating stations with a common time reference that can be delivered for example by signals picked up by receivers and transmitted by a satellite positioning network of a well-known type such as the GPS system or the carrier frequency of a radio transmitter, and in using this common reference to calculate the real transmission time and to locally readjust the seismic acquisition time break.
The transmission method according to the invention transfers, via a standard communication network (LAN), a synchronization signal indicating a time break (TB) between a central control and seismic recording station (CCU) and seismic signal acquisition and storage units (RTU) distributed in the field, when the time of transit of the synchronization signal through at least part of the connection channels of the standard network between the central station and at least one dependent station (which can be an intermediate station (RRS), itself connected by cables or fibres to acquisition units in the field (RTU) or possibly each acquisition unit in configurations where they directly controlled by the central station) may be liable to fluctuations within a determined fluctuation margin.
The method comprises:
a learning stage including formation of a specific frame and storage thereof by each station, and detection in each dependent station of signals delivered by a clock exterior to the device,
pre-initiation of the acquisition of seismic data by acquisition units (RTU) with a retention time interval in relation to the time of reception of synchronization signal (TB) which is fixed and greater than the maximum transit time, considering the fluctuation margin,
first precise dating of time break (TB) from the external clock and transmission of the data obtained with this first dating in form of the specific frame to the dependent stations,
detection of the time of arrival, in each dependent station, of the specific frame and second dating of this time of arrival from the external clock, and
measurement of the effective transit time equal to the time interval between the data associated with the first dating and those associated with the second dating, and consequently readjustment of the time break to the seismic data stored in acquisition units (RTU).
The pre-initiation time is for example the time of transmission to a seismic source (S) of a fire order.
In the case where the (or each) dependent station is a local station (RRS) connected to at least one acquisition unit (RTU) by a material link with a fixed transit time, and to central station (CCU) by a fluctuating transit time link, a readjustment to the seismic data stored in acquisition units (RTU) is applied by taking into account the fixed transit time.
The method comprises for example:
direct detection, in central station (CCU), of synchronization signal (TB),
formation, from the synchronization signal detected, of the specific frame and direct application thereof to an interface module (TCI) of central station (CCU),
detection, in an interface module (TCI) of each dependent station, of a specific frame of synchronization signal (TB), and
application, to synchronization signal (TB) of the dependent station, of a time lag equal to the measured transit time.
The method is particularly flexible by allowing, by means of an adjustable transmission delay time compensation, easily taking into account the various data transfer rates of the transmission channels available in a complex transmission system as used for example in modem seismic prospecting devices.
The transmission system according to the invention transfers, by means of a standard communication network (LAN), a synchronization signal indicating a time break (TB) between a central control and seismic recording station (CCU) and seismic data acquisition and storage units (RTU) distributed in the field, when the time of transit of the synchronization signal through at least part of the connection channels of the standard network between the central station and at least one dependent station may be liable to fluctuations within a determined fluctuation margin. The system comprises:
means for forming a specific frame, storage means allowing each element of the seismic device to store this specific frame, and a local clock (RXGPS) controlled by synchronization signals provided by the external clock (H) in order to generate a dating time scale, the local clock being associated with the elements of the seismic device,
means for pre-initiating, in acquisition units (RTU), acquisition of the seismic data with a retention time interval in relation to the time of reception of synchronization signal (TB) that is fixed and greater than the maximum transit time, considering the fluctuation margin,
counting means associated with each local clock so as to perform a first precise dating of time break (TB) in accordance with the external clock and transmission means for transmitting the data obtained with this first dating in form of the specific frame to the dependent stations,
means for detecting the time of arrival, at each dependent station, of the specific frame and counting means associated with the local clock for performing a second dating of the time of arrival in accordance with external clock (H), and
counting means for determining the effective transit time equal to the time interval between the data associated with the first dating and those associated with the second dating, and means for accordingly readjusting the time break to the seismic data stored in acquisition units (RTU).
The system comprises for example an interface set (TBG/I) in a central station (CCU) for directly generating on a transmission channel, to each dependent station, a frame carrying the data from the first dating, an interface set (TBD/I) for decoding the specific frame, counting means (D-CPT) for performing the second dating in accordance with external clock (H) and for determining the effective transit time of the synchronization signals.
In the case where the (or each) terminal station is a local station (RRS), a generator (SYNCTB-G) for generating a signal (SYNCTB) synchronous with the delayed signal to acquisition units (RTU).
The common external clock is for example synchronization signals emitted by a satellite positioning system and, each element of the seismic device comprises a suitable detection module.
The common external clock can also be for example of synchronization signals emitted by a radio transmitter and, each element of the seismic device comprises a suitable detection module.
The combination of a fixed retention time applied by all the acquisition units and of intermediate delay adjustment means suited to complete the propagation times measured on the transmission channels by reference to an external clock allows all the acquisition units to be adjusted together to the same time break.
Whether the time of transmission by the network is known with precision or not, it is possible to combine such a network with conventional dedicated transmission channels using optical fibres, radio or cable links, while maintaining a perfect synchronism between these various channels.